1. Field of the Invention
The present invention concerns an analog-digital converter. It is designed to work at microwave frequencies at very high conversion speeds, and converts an analog signal into a precision digital signal with a large number of bits, through three stages only. The present invention requires only a small number of transistors, most of which work in parallel and do not affect the conversion time. The input signal, addressed to as many comparator circuits as there are bits in the output signals, is separated from it only by two gate-drain junctions of two transistors. The comparators work in cascade.
This converter converts, firstly, the analog input voltage V.sub.E into an analog current I.sub.E then, for each output bit level, it compares this current I.sub.E with a calibrated current nI.sub.o. The resulting positive or negative current is addressed to a logic circuit, a first output of which partly controls the comparators of all the levels of less signficant bits and a second output of which gives a bit of the digital signal.
The fact that the comparators are partially controlled can be explained thus: each comparator is formed by a series of elementary current sources, which are parallel mounted and deliver currents in geometrical progression 2.sup.n : 1, 2, 4, 8 . . . , with a transistor as the switch. In a series, such as this, of current sources, some of them may be off, depending on the state of the first output of the logic circuit which controls them, while the others deliver a fixed current.
Furthermore, since each comparator is formed by several sources of elementary currents, the first output of a logic circuit which gives a level n bit controls all the elementary current sources, putting through a current 2.sup.n I.sub.o, of all the less significant bit levels. There is thus cascading between the conversion levels.